1. Field of the Invention
The present invention relates to an organic light emitting display device and a method of driving the same.
2. Description of Related Art
In recent years, various flat panel display devices have been developed which are lightweight and smaller when compared to cathode ray tubes (CRTs). Flat display panels include liquid crystal display (LCD) devices, field emission display (FED) devices, plasma display panels (PDPs), and organic light emitting display devices among others.
Among the flat display panels, the organic light emitting display device (or OLED display device) displays an image by using organic light emitting diodes (OLEDs) that generate light by means of recombination of electrons and holes. The organic light emitting display device is advantageous in that it has a fast response time, and is driven with low power consumption.
FIG. 1 is a circuit diagram showing a pixel in a conventional organic light emitting display device.
Referring to FIG. 1, the pixel 4 of the organic light emitting display device includes an organic light emitting diode (OLED) and a pixel circuit 2 coupled to a data line (Dm) and a scan line (Sn) to control the organic light emitting diode (OLED).
An anode electrode of the organic light emitting diode (OLED) is coupled to the pixel circuit 2, and a cathode electrode of the organic light emitting diode (OLED) is coupled to a second power source (ELVSS). The organic light emitting diode (OLED) generates light with a brightness corresponding to an electric current supplied from the pixel circuit 2.
The pixel circuit 2 controls an amount of current supplied to the organic light emitting diode (OLED) corresponding to a data signal supplied to the data line (Dm) when a scan signal is supplied to the scan line (Sn). For this purpose, the pixel circuit 2 includes a second transistor (M2) coupled between a first power source (ELVDD) and the organic light emitting diode (OLED), a first transistor (M1) coupled between the data line (Dm) and a gate electrode of the second transistor (M2), and a storage capacitor (C) coupled between the gate electrode and a first electrode of the second transistor (M2).
A gate electrode of the first transistor (M1) is coupled to the scan line (Sn), and a first electrode of the first transistor (M1) is coupled to the data line (Dm). A second electrode of the first transistor (M1) is coupled to a first terminal of the storage capacitor (C). Here, the first electrode is one of a source electrode and a drain electrode, and the second electrode is the other of the source electrode and the drain electrode. For example, when the first electrode is a source electrode, the second electrode is a drain electrode. When a scan signal is supplied from the scan line (Sn), the first transistor (M1) is turned on to supply a data signal from the data line (Dm) to the storage capacitor (C). In this case, the storage capacitor (C) charges a voltage corresponding to the data signal.
A gate electrode of the second transistor (M2) is coupled to the first terminal of the storage capacitor (C), and a first electrode of the second transistor (M2) is coupled to a second terminal of the storage capacitor (C) and the first power source (ELVDD). A second electrode of the second transistor (M2) is coupled to an anode electrode of the organic light emitting diode (OLED). The second transistor (M2) controls an amount of current corresponding to a voltage stored in the storage capacitor (C), the current being supplied from the first power source (ELVDD) to the second power source (ELVSS) via the organic light emitting diode (OLED). In this case, the organic light emitting diode (OLED) generates light corresponding to the current supplied from the second transistor (M2).
The pixel 4 of the organic light emitting display device displays an image by repeating the above-mentioned operations. Meanwhile, the first power source (ELVDD) and the second power source (ELVSS) are supplied to the organic light emitting diode (OLED) in a digital driving mode in which the second transistor (M2) functions as a switch, and therefore the organic light emitting diode (OLED) is driven at a constant voltage to emit light. Such a digital driving mode is advantageous in that the organic light emitting display device may display an image regardless of a non-uniform threshold voltage of the second transistor (M2).
However, since a constant voltage is applied to the organic light emitting diode (OLED) in the digital driving mode, the organic light emitting diode (OLED) may be rapidly degraded, therefore making it very difficult to display an image with uniform brightness.